Because of their capabilities for efficient removal of charged substances or uncharged ones, semipermeable membranes have been utilized in wide-ranging areas, including substances of desalination of seawater and brackish water, production of pure water and ultrapure water for medical and industrial uses, wastewater treatment and food industries. In separation treatment using the semipermeable membranes, a membrane element using a semipermeable membrane has been generally adopted.
FIG. 1 is a schematic cross-sectional view showing one embodiment of a membrane separation system using semipermeable membrane elements. A plurality of semipermeable membrane elements 1 are loaded in series in a pressure vessel 2, and water to be treated (raw water) is fed into the pressure vessel 2 in a state of being pressurized by a pump 3 and separated into concentrate and permeate.
The water quality of permeate obtained in such a membrane separation system is generally affected by concentrations of impurities contained in raw water. In addition, with regard to some of the impurities, when they are excessively concentrated in the vicinity of the surfaces of semipermeable membrane, a phenomenon like fouling or scaling is caused to result in clogging of the semipermeable membranes. The term “fouling” as used herein refers to the phenomenon in which suspended substances, colloidal substances, organic substances and the like in water are deposited or adsorbed on membrane surfaces, and the term “scaling” as used herein refers to the phenomenon in which dissolved salts in water are precipitated out and deposited on membrane surfaces when the water is concentrated to an extent beyond solubility limits of the dissolved salts.
When concentration degrees of impurities become high in the vicinity of the surfaces of semipermeable membranes, there occur increases in amounts of impurities leaked into permeate, thereby causing a problem that the permeate suffers deterioration in water quality. In the membrane separation system, it therefore becomes necessary to maintain the removal ratio of impurities within a predetermined range and suppress deterioration in water quality of permeate by operating the system so as not to cause concentration to a higher degree than necessary in the vicinity of the surfaces of semipermeable membranes. In addition, when there occurs a clogging of semipermeable membranes in the semipermeable membrane elements, permeation flux of water is reduced, and a flow rate of permeate is lowered. In the membrane separation system, there is therefore a necessity not only to maintain the removal ratio of impurities within a predetermined range and suppress the deterioration in water quality of permeate by operating the system so as not to cause concentration to a higher degree than necessary in the vicinity of surfaces of semipermeable membranes, but also to control a decrease in flow rate of permeate through the prevention of fouling and scaling. Further, with the flow rate of permeate usually kept constant by varying a pressure (operation pressure) for pressurization of the water to be treated during the operation of the membrane separation system, the excessive concentration occurring in the vicinity of semipermeable membrane surfaces causes increase of impurity concentrations in the vicinity of the semipermeable membrane surfaces to result in a rise of osmosis pressure, whereby the operation pressure is heightened and energy loss is caused. Thus it is necessary in the membrane separation system to perform efficient operations by operating the system so as not to cause concentration to a higher degree than necessity in the vicinity of the surfaces of semipermeable membranes.
As a control index to prevention of excessive concentration during operations of the membrane separation system, a recovery rate is adopted. The term “recovery rate” as used herein means that, in a case e.g. shown in FIG. 2 where the flow rate of water to be treated in a membrane filtration section 4 provided with a semipermeable membrane element is 100 L/min and the flow rate of permeate is 20 L/min, the recovery rate is calculated from the proportion of the flow rate of permeate to the flow rate of water to be treated, and it becomes 20÷100×100=20%. Additionally, the flow rate of water to be treated in the membrane separation system is the sum of the flow rate of concentrate and the flow rate of permeate, and the recovery rate is generally adjusted by measuring the flow rate of concentrate and the flow rate of permeate with a concentrate flow meter 5 and a permeate flow meter 6, respectively, and by manipulating valves for control of their respective flow rates.
In a case where the recovery rate is a high value, it is thought that excessive concentration occurs in the vicinity of the surface of the semipermeable membrane and, as mentioned above, there are possibilities of causing deterioration in water quality of permeate of the semipermeable membrane, lowering the flow rate of permeate and occurring an energy loss. On the other hand, in a case where the recovery rate is a low value, there arises a problem that the ratio of the flow rate of concentrate to the flow rate of raw water becomes high; as a result, the concentrate is discharged to excess, and it becomes difficult to aim at effective use of water.
It is therefore necessary to carry out operations free from such excessive concentration and excessive discharge of concentrate as mentioned above by setting an ideal recovery rate according to preconditions including the kind of water to be treated, the quality of water to be treated, water temperature and so on and operating the membrane separation system so as to achieve the ideal recovery rate.
As a membrane filtration method capable of maintaining both the flow rate of permeate and the water quality of permeate, for example, there has been known the method disclosed in Patent Document 1. In the Patent Document 1, it has been suggested that the amount of concentrate to be discharged should be adjusted according to either the water temperature of any of water to be treated, permeate and concentrate or the quality of water to be treated, and besides, the amount of concentrate to be discharged should be adjusted based on the condition of deterioration in water quality of permeate, and the clogging and wearing conditions of a filtration membrane.